Plants must adapt themselves to various stresses, for example, drought, salt in the soil, and low temperature because they can not move freely. Among these stresses, drought is thought to effect plant growth the most severely. In order to survive in drought condition, some plants have acquired a physiologically and/or morphologically specific trait in the evolutional process while many other plants also confer a mechanism to response to the drought stress and defend themselves. These responses to a shortage of water and adaptation to drought environment in plants are caused by various physiological changes including the alternation of gene expression at drought (Shinozaki, K and Yamaguchi-Shinozaki, K., Plant Physiol., 115: 327-334, 1997; Shinozaki, K. and Yamaguchi-Shinozaki, K., “Molecular responses to drought stress.” In Shinozaki and Yamaguchi-Shinozaki (eds), “Molecular responses to cold, drought, heat and salt stress in higher plants,” R. G. LANDES company, Austin, Tex., USA, pp. 11-28, 1999). For example, in Arabidopsis (Arabidopsis thaliana), it is known that a drought signal is transmitted through an abscisic acid (ABA) dependent pathway and ABA independent pathway to control the gene expression involved in drought tolerance. These gene products are thought to have a function in controlling, for example, accumulation of osmoprotectants such as sucrose and proline, half life of proteins, stress signal transduction pathway, and transcription (Bray, E. A., Trends in Plant Science, 2: 48-54, 1997; Bohnert, H. J. et al., Plant Cell, 7: 1099-1111, 1995; Ingram, J. and Bartels, D., Annu. Rev. Plant Physiol. Plant Mol. Biol., 47: 377-403, 1996; Shinozaki, K. and Yamaguchi-Shinozaki, K., Plant Physiol., 115: 327-334, 1997; Shinozaki, K. and Yamaguchi-Shinozaki, K., “Molecular responses to drought stress.” In Shinozaki and Yamaguchi-Shinozaki (eds), “Molecular responses to cold, drought, heat and salt stress in higher plants,” R. G. LANDES company, Austin, Tex., USA, pp. 11-28, 1999).
C40 pathway has been proposed as a biosynthetic pathway of ABA in higher plants. The C40 pathway, also called a carotenoid pathway, is a synthetic pathway through epoxydation of zeaxanthin, synthesizing violaxanthin, neoxanthin, xanthoxin, ABA aldehyde, and then ABA (Zeevaart, J. A. D. and Creelman R. A., Ann. Rev. Plant Physiol. Plant Mol. Biol., 39: 439-473, 1988). This biosynthetic pathway has been proposed from physiological studies and analyses of ABA biosynthetic variants. For example, variant aba2 isolated from tobacco (Nicotiana tabacum) has a mutation in a gene (aba2) of zeaxanthin epoxidase enzyme which catalyzes the epoxidation of zeaxanthin (Marin E. et al., EMBO J., 15: 2331-2342, 1996). Variant vp14 isolated from maize has a mutation in a gene (VP14) of neoxanthin cleavage enzyme which catalyzes the conversion from a neoxanthin to xanthoxin (Tan, B. C. et al., Proc. Natl. Acad. Sci. USA, 94: 12235-12240, 1997). From Arabidopsis plants, variant aba3 having a mutation in an enzyme which catalyzes a reaction from xanthoxin to ABA aldehyde, and variant aba4 involved in the reaction for oxidizing ABA aldehyde to produce ABA have been isolated (Schwartz, S. H. et al., Plant Physiol., 114: 161-166, 1997; Leon-Kloosterziel, K. M. et al., Plant J., 10: 655-661, 1996).
A maize having a mutation in a neoxanthin cleavage enzyme gene (VP14) is known to show a trait of easily loosing water and easily wilting. It has not been known yet, however, whether stress tolerance in plants can be improved or not using the neoxanthin cleavage enzyme gene.